This is the most ever for the series dating back to its inception in 2001. The jump in job openings also came as hiring and firing declined slight in April, indicating that the labor market continues to tighten as the available pool of workers has been taken up during the post-crisis economic expansion.

Friday’s report also showed that those moving from employed to not in the labor force altogether hit its second-highest monthly total since the financial crisis, showing that the trend of Baby Boomers retiring continues to drive down the total size of the labor force. As the labor force decreases in size, the labor force participation rate also falls, bringing down the unemployment rate as U.S. companies continue to add workers.

Recent economic surveys, notably the Institute for Supply Managements manufacturing and non-manufacturing business surveys for May, have shown that labor continues to be a persistent concern for American businesses as good workers are in short supply. The Federal Reserve’s latest Beige Book report also indicated that across the country business contacts were having a hard time staffing up due to a lack of qualified workers.

Jed Kolko, chief economist at Indeed, noted on Tuesday that the latest JOLTS number brought down two often overlooked measures of labor market tightness, or signs that the balance of power in the labor market is shifting from employers to employees.